All brains age – but how they do so differs among us all in singular ways. Some wane in cognition and others remain sharp even as they idle up in years. Now, it seems, gender may play a role too.

Women’s brains, on average, appear more than three years younger than men’s of the same age, at least in terms of metabolic rate. A team from the Washington University School of Medicine in St. Louis used the brain's metabolic activity to determine age based on previous studies that found aging is associated with a decline in such activity. The research is published in the Proceedings of the National Academy of Sciences.

For the study, the researchers used PET technology to measure the flow of oxygen and glucose in the brains of more than 200 individuals (121 women, 84 men) ranging in age from 20 to 82 years. For each participant, they noted how much sugar certain regions of their brain devoted to aerobic glycolysis – a metabolic process. A machine-learning algorithm was then tasked with finding a relationship between their age and brain metabolism. First, the team taught the algorithm to predict the age of men from their metabolic data. The program estimated men’s ages correctly, but when fed the women’s data, they appeared to be 3.8 years younger than men’s brains.

Intrigued, the team swapped the analysis around and trained the machine to predict women’s ages from metabolic data. When they did it this way, the men were estimated to be 2.4 years older than their actual age. Even more curious, this metabolic gap between men and women was seen even in their 20s.

"It's not that men's brains age faster – they start adulthood about three years older than women, and that persists throughout life," said senior author Manu Goyal, an assistant professor of radiology at the university's Mallinckrodt Institute of Radiology, in a statement. "What we don't know is what it means. I think this could mean that the reason women don't experience as much cognitive decline in later years is because their brains are effectively younger, and we're currently working on a study to confirm that."

It should be emphasized that the participants' brain age here was only determined by metabolic activity, not other factors. Whether other physiological differences exist was not studied in this research. However, it does open doors for further study to see if this contributes to the trend we see in female resiliency to age-related changes.

"The average difference in calculated brain age between men and women is significant and reproducible, but it is only a fraction of the difference between any two individuals," Goyal said. "It is stronger than many sex differences that have been reported, but it's nowhere near as big a difference as some sex differences, such as height."

For now, more research is needed to better understand why this difference occurs and whether it plays a role in age-related diseases such as Alzheimer’s. A key follow-up study would see whether low glucose metabolism in the brain makes an individual more prone to cognition declines and neurodegenerative diseases.

"We're just starting to understand how various sex-related factors might affect the trajectory of brain aging and how that might influence the vulnerability of the brain to neurodegenerative diseases," added Goyal. "Brain metabolism might help us understand some of the differences we see between men and women as they age.”